backward compatibility

Able to share data or commands with older versions of
itself, or sometimes other older systems, particularly systems
it intends to supplant. Sometimes backward compatibility is
limited to being able to read old data but does not extend to
being able to write data in a format that can be read by old
versions.

For example, WordPerfect 6.0 can read WordPerfect 5.1 files,
so it is backward compatible. It can be said that Perl is
backward compatible with awk, because Perl was (among other
things) intended to replace awk, and can, with a converter,
run awk programs.